[SPARK-17840][DOCS] Add some pointers for wiki/CONTRIBUTING.md in README.md and some warnings in PULL_REQUEST_TEMPLATE

## What changes were proposed in this pull request?

Link to contributing wiki in PR template, README.md

## How was this patch tested?

Doc-only change, tested by Jekyll

Author: Sean Owen <sowen@cloudera.com>

Closes #15429 from srowen/SPARK-17840.